1. Field of the invention
The present invention relates to an organic EL device and a method of manufacturing the same.
2. Description of Related Art
Recently, in organic EL devices, active matrix organic light emitting diodes (AM-OLED) that can individually control respective pixels are widely used. Each of the pixels typically includes a first thin film transistor (TFT) and a second TFT formed on a transparent substrate, a transparent lower electrode electrically connected to a storage capacitor and a drain electrode of the second TFT, an organic EL region formed on the lower electrode to emit light having a predetermined wavelength, and an opaque upper electrode formed of a material such as aluminum over the whole surface of the transparent substrate.
At this point, let us define the AM-OLED that emits light through a back surface of the organic EL region due to the opaque upper electrode as a “back surface emitting AM-OLED.” A surface through which light emits as a “back surface” and a surface through which light does not emit is defined as a “front surface.”
Meanwhile, in the back surface emitting AM-OLED, the pixel area is mostly occupied by the first and second TFT and the storage capacitor. Therefore, light emitted from just about 20% of total pixel area is directed toward observers. In other words, the back surface emitting AM-OLED has an aperture ratio of about 20%. It makes it difficult to achieve a high luminance. More current has to be applied, in order to increase brightness in a low aperture ratio device. This increases power consumption, which is unsuitable for portable display devices.
In order to overcome the problems, the AM-OLED should have a front surface emitting structure which light is emitted through the front surface. In the front surface emitting AM-OLED, the lower electrode should be made of an opaque material, and the upper electrode should be made of a transparent material.
However, if the transparent upper electrode is formed on the organic EL region, the relatively high temperature required for depositing the transparent upper electrode damages, the organic EL region. Therefore, it is very difficult to produce a front surface emitting AM-OLED.
U.S. Pat. No. 6,046,543 to Bulovic, et al., entitled “high reliability, high efficiency, integratable organic light emitting devices and methods of producing same,” describes a method of forming a transparent cathode electrode in order to implement the front surface emitting AM-OLED. U.S. Pat. No. 5,981,306 to Burrows, et al., entitled “method for depositing indium tin oxide layers in organic light emitting devices,” describes a method of forming indium tin oxide (ITO) at a very low depositing rate in order to implement the front surface emitting AM-OLED. U.S. Pat. No. 5,739,545 to Guha, et al., entitled “organic light emitting diodes having transparent cathode structures,” describes the front surface emitting AM-OLED implemented using a transparent cathode electrode.
However, such methods are not easy to implement, and demand a long processing time. In addition, it is difficult to control device characteristics. Besides, in the conventional AM-OLED, as described above, the metal can is used to protect the organic EL region from oxygen and moisture, thus increasing the weight and volume of the AM-OLED as a whole.